2-D Spring-Damper
Bearing
COMBI214 Element Description
COMBI214 has longitudinal as well as cross-coupling capability in 2-D applications. It is a tension-compression element with up to two degrees of freedom at each node: translations in any two nodal directions (x, y, or z). COMBI214 has two nodes plus one optional orientation node. No bending or torsion is considered.
For user-defined characteristics (KEYOPT(1) = 0), the spring, damping, or mass capability may be removed from the element.
When used as a cylindrical journal bearing or squeeze film damper (KEYOPT(1) ≠ 0), the fluid film pressure forces based on finite length assumption are calculated. Only static and nonlinear transient analyses are supported. Static analysis can be used to determine pressure forces at a known equilibrium position. Stiffness/damping characteristics can also be obtained by specifying a perturbation increment in the element real constants.
A longitudinal spring/damper with torsion capabilities is available via the COMBIN14 element. A general spring or damper is also available in the stiffness, damping or mass matrix element MATRIX27. Another spring-damper element having its direction of action determined by the nodal coordinate directions is COMBIN40.
For more information about this element, see COMBI214 - 2-D Spring-Damper Bearing in the Mechanical APDL Theory Reference.
Figure 214.2: COMBI214 Geometry for KEYOPT(1) > 0 and KEYOPT(2) = 0
I is attached to the bearing journal.
J is attached to the rotor.
COMBI214 Input Data
The geometry, node locations, and coordinate system for this element are shown in Figure 214.1: COMBI214 Geometry for KEYOPT(1) = 0 and KEYOPT(2) = 0. The element is defined by two nodes.
Depending on the element technology (KEYOPT(1)) specified, the inputs involve stiffness characteristics K11, K22, K12 and K21, damping characteristics C11, C22, C12 and C21, and mass characteristics M11, M22, M12, and M21 or bearing geometry data.
KEYOPT(2) = 0 through 2 options define the element plane. The element operates in the nodal coordinate system. The nodal coordinate systems of nodes I and J must be the same. (1) and (2) indicate the first and second axis of the element plane defined by KEYOPT(2). For example, if KEYOPT(2) = 0 and there is no nodal coordinate system defined for nodes I and J, then (1) is the global X-axis and (2) is the global Y-axis.
A summary of the element input is given in "COMBI214 Input Summary". A general description of element input is given in Element Input.
User-Defined Stiffness/Damping Characteristics (KEYOPT(1) = 0)
The stiffness coefficients should have units of Force/Length, and the damping coefficient units are Force*Time/Length. The damping capability is not used for static or undamped modal analyses. The mass coefficients should have units of Force*Time2/Length.
For stiffness, damping, and mass real constants,
either numerical values or tabular array inputs can be specified. If specifying tabular inputs,
enclose the table name within % characters (%tabname%).
These real constants can vary with the amplitude of the rotational velocity vector (defined via the OMEGA or CMOMEGA command). Use the *DIM command and the primary variable OMEGS to dimension the table and identify the variable. Because the amplitude of the rotational velocity vector is an absolute value, only positive values of OMEGS in the table parameter are valid.
Stiffness and damping real constants can also vary with the eccentricity (the position of the center line of the rotor) and/or the phase shift between the displacements in the two nodal directions. Use the *DIM command and the primary variable ECCENT and/or THETA to dimension the table and identify the variables. The phase shift values unit (THETA) is the degree. ECCENT and THETA only apply to nonlinear analyses.
For more information about using tabular inputs, see Array Parameters in the ANSYS Parametric Design Language Guide, Applying Loads Using TABLE Type Array Parameters in the Basic Analysis Guide, and Performing a Thermal Analysis Using Tabular Boundary Conditions in the Thermal Analysis Guide.
The KEYOPT(3) = 0 and 1 options specify whether or not the element is symmetric. When symmetric, cross-coupling terms in stiffness, damping, and mass coefficients are equal (that is, K12 = K21, C12 = C21, and M12 = M21).
Reynolds Equation Integration (KEYOPT(1) = 1 or 2)
Calculating all element results (KEYOPT(1) = 2) may be computationally expensive. KEYOPT(1) = 1 can be used to calculate a limited number of element outputs. See Table 214.2: COMBI214 Element Output Definitions for KEYOPT(1) = 1 or 2.
The bearing geometry is defined by real constants: radial clearance (C), length (L), and rotor radius (R).
In a static analysis, the location of the rotor center is specified using degree of freedom constraints (D command) and the translational velocities of the rotor are defined by real constants (Veloc1 and Veloc2). To obtain the stiffness and damping characteristics, the non-dimensional perturbation increment (real constant PertInc) must be specified along with KEYOPT(1) = 2.
The integration of Reynolds equations is performed with a circumferential step of 2 degrees. It can be modified using real constant ThetaInc.
In the particular case of a squeeze film damper in synchronous precession (when the rotor center describes an orbit having the same frequency as the rotational velocity), the real constant OmgPrec is used.
COMBI214 Input Summary
- Nodes
I, J
- Degrees of Freedom
UX, UY (KEYOPT (2) = 0) UY, UZ (KEYOPT (2) = 1) UX, UZ (KEYOPT (2) = 2) - Real Constants
For user-input stiffness, damping, and mass characteristics (KEYOPT(1) = 0):
K11, K22, K12, K21 , C11, C22, C12, C21, M11, M22, M12, M21 Kij - (i=1,2 j=1,2) Stiffness coefficients Cij - (i=1,2 j=1,2) Damping coefficients Mij - (i=1,2 j=1,2) Mass coefficients. See KEYOPT(6) for mass location. Note: All real constants may be defined as table parameters function of the rotational velocity (using primary variable OMEGS). The stiffness and damping real constants may be defined as table parameters function of the eccentricity (using primary variable ECCENT), and/or the phase shift (using primary variable THETA). If the rotational velocity is the only parameter, the stiffness and damping characteristics can be imported directly from an ASCII text file using the APDL macro importbearing1.mac. See Selecting Parts and Bearings for more details.
For integration of Reynolds equations (KEYOPT(1) = 1 or 2):
C, L, R, Veloc1, Veloc2, PertInc, ThetaInc, OmgPrec
See "COMBI214 Input Data (KEYOPT(1) = 1 or 2)" for a detailed description of these real constants.
- Material Properties
None for KEYOPT(1) = 0
MP, VISC for KEYOPT(1) = 1 or 2
- Surface Loads
None
- Body Loads
None
- Special Features
Birth and death Large deflection Linear perturbation Stress stiffening - KEYOPT(1)
Type of element technology:
- 0 --
User-defined characteristics. The value option is the default.
- 1 --
Cylindrical journal bearing - integration of Reynolds equation to calculate the pressure forces.
- 2 --
Cylindrical journal bearing - integration of Reynolds equation to calculate the fluid film thickness, pressure, and pressure forces.
- KEYOPT(2)
Degrees of freedom selection:
- 0 --
Element lies in a plane parallel to the XY plane. The degrees of freedom are UX and UY. This value option is the default.
- 1 --
Element lies in a plane parallel to the YZ plane. The degrees of freedom are UY and UZ.
- 2 --
Element lies in a plane parallel to the XZ plane. The degrees of freedom are UX and UZ.
- KEYOPT(3)
Symmetry for user-input characteristics (KEYOPT(1) = 0):
- 0 --
Element is symmetric: K12 = K21, C12 = C21, and M12 = M21. This option is the default.
- 1 --
Element is not symmetric.
- KEYOPT(4)
Element matrices output for user-input characteristics (KEYOPT(1) = 0):
- 0 --
Do not print element matrices.
- 1 --
Print element matrices at beginning of solution phase.
- KEYOPT(6)
Mass location (KEYOPT(1) = 0):
- 0 --
No mass.
- 1 --
Mass at node J.
- 2 --
Mass equally distributed between nodes I and J.
- 3 --
Mass at node I.
COMBI214 Input Data (KEYOPT(1) = 1 or 2)
| No. | Name | Description |
|---|---|---|
| 1 | C | Radial clearance |
| 2 | L | Bearing length |
| 3 | R | Radius of the rotor |
| 4 | Veloc1 | Velocity along direction (1) - used in static analysis only (ANTYPE,STATIC). |
| 5 | Veloc2 | Velocity along direction (2) - used in static analysis only (ANTYPE,STATIC). |
| 6 | PertInc | Nondimensional perturbation increment for stiffness and damping characteristics calculation. The actual increment is PertInc * C. Defaults to 0 and the characteristics are not calculated. |
| 7 | ThetaInc | Theta increment in degrees for pressure integration. Defaults to 2 degrees. |
| 8 | OmgPrec | Synchronous precession rotational velocity for the calculation of the squeeze film damper characteristics. If nonzero, the rotational velocity must be zero, Veloc1 and Veloc2 are ignored, and PertInc must be nonzero. |
COMBI214 Output Data
The solution output associated with the element is in two forms:
Nodal displacements included in the overall nodal solution
Additional element output as shown in Table 214.1: COMBI214 Element Output Definitions for KEYOPT(1) = 0.
The Element Output Definitions table uses the following notation:
A colon (:) in the Name column indicates the item can be accessed by the Component Name method [ETABLE, ESOL]. The O column indicates the availability of the items in the file Jobname.OUT. The R column indicates the availability of the items in the results file.
In either the O or R columns, Y indicates that the item is always available, a number refers to a table footnote that describes when the item is conditionally available, and a - indicates that the item is not available.
Table 214.1: COMBI214 Element Output Definitions for KEYOPT(1) = 0
| Name | Definition | O | R |
|---|---|---|---|
| EL | Element Number | Y | Y |
| NODES | Nodes - I, J | Y | Y |
| XC, YC, ZC | Location where results are reported | Y | 1 |
| FORC1 | Spring force along direction (1) | Y | Y |
| FORC2 | Spring force along direction (2) | Y | Y |
| STRETCH1 | Stretch of spring along direction (1) | Y | Y |
| STRETCH2 | Stretch of spring along direction (2) | Y | Y |
| VELOCITY1 | Velocity along direction (1) | - | Y |
| VELOCITY2 | Velocity along direction (2) | - | Y |
| DAMPING FORCE1 | Damping force along direction (1) -- Zero unless this is a transient analysis (ANTYPE,TRANS) and damping is present | Y | Y |
| DAMPING FORCE2 | Damping force along direction (2) -- Zero unless this is a transient analysis (ANTYPE,TRANS) and damping is present | Y | Y |
Table 214.2: COMBI214 Element Output Definitions for KEYOPT(1) = 1 or 2
| Name | Definition | O | R |
|---|---|---|---|
| EL | Element number | Y | Y |
| NODES | Nodes - I, J | Y | Y |
| XC, YC, ZC | Location where results are reported | Y | 1 |
| FORC1 [2] | Pressure force along direction (1) | Y | Y |
| FORC2 [2] | Pressure force along direction (2) | Y | Y |
| THETA1 | Starting angle for positive pressures | Y | Y |
| THETA2 | Ending angle for positive pressures | Y | Y |
| MOFP [3] | Maximum of fluid pressure | Y | Y |
| THETAP [3] | Angle of maximum fluid pressure | Y | Y |
| MOFT [3] | Minimum of fluid thickness | Y | Y |
| THETAT [3] | Angle of minimum fluid thickness | Y | Y |
| K11 [4] | Stiffness coefficient | Y | Y |
| K22 [4] | Stiffness coefficient | Y | Y |
| K12 [4] | Stiffness coefficient | Y | Y |
| K21 [4] | Stiffness coefficient | Y | Y |
| C11 [4] | Damping coefficient | Y | Y |
| C22 [4] | Damping coefficient | Y | Y |
| C12 [4] | Damping coefficient | Y | Y |
| C21 [4] | Damping coefficient | Y | Y |
Available only at centroid as a *GET item.
Forces are opposite to the fluid forces acting on the rotor.
Available for KEYOPT(1) = 2 only when the perturbation increment (PertInc) is nonzero.
Table 214.3: COMBI214 Item and Sequence Numbers for KEYOPT(1) = 0 lists output available via the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table in this document for more information. The following notation is used in Table 214.3: COMBI214 Item and Sequence Numbers for KEYOPT(1) = 0:
- Name
Output quantity as defined in Table 214.1: COMBI214 Element Output Definitions for KEYOPT(1) = 0
- Item
Predetermined Item label for the ETABLE command
- E
Sequence number for single-valued or constant element data
Output data for COMBI214 consists of the following:
COMBI214 Assumptions and Restrictions
Nodes must lie in the plane defined by KEYOPT(2).
The element allows only a uniform stress in the springs.
When KEYOPT(1) = 0:
K11 and K22 must be non-zero to activate the spring capability.
C11 and C22 must be non-zero to activate the damper capability.
M11 or M22 must be non-zero to activate the mass capability.
The following applies when KEYOPT(3) = 0 (symmetric):
If K12 is non-zero and K21 is zero, then K21 is set to K12.
If C12 is non-zero and C21 is zero, then C21 is set to C12.
If M12 is non-zero and M21 is zero, then M21 is set to M12,
The spring, damping, or mass capability may be deleted from the element by setting all Kij (i=1,2 j=1,2), all Cij (i=1,2 j=1,2), or all Mij (i=1,2 j=1,2) equal to zero, respectively.
Rotating damping effect (RotDamp = ON in the CORIOLIS command) is taken into account if the diagonal damping characteristics (C11 and C22) are equal and non-zero and cross-terms (C12 and C21) are zero.
Rotating damping effect is ignored if the element has a real constant input as a table array.
The degrees of freedom are specified in the nodal coordinate system and are the same for both nodes. (For more information, see Elements That Operate in the Nodal Coordinate System.) If the nodal coordinate systems are rotated relative to each other, the same degree of freedom may be in different directions (thereby giving possibly unexpected results).
No moment effects are included; that is, if the nodes are offset from the lines of action, moment equilibrium may not be satisfied.
The element is defined such that a positive displacement of node J relative to node I tends to stretch the spring. If, for a given set of conditions, nodes I and J are interchanged, a positive displacement of node J relative to node I tends to compress the spring.
COMBI214 Product Restrictions
When used in the product(s) listed below, the stated product-specific restrictions apply to this element in addition to the general assumptions and restrictions given in the previous section.
ANSYS Mechanical Pro
KEYOPT(1) is set to 0 and cannot be changed.
Real constants Veloc2, PertInc, ThetaInc, OmgPrec default to 0 and cannot be changed.
Damping is not available (real constants C11, C22, C12, C21 default to 0 and cannot be changed).
Birth and death is not available.
Linear perturbation is not available.
ANSYS Mechanical Premium
Birth and death is not available.